Even if you think you know all the tropes, a particularly clever anime can still hit you with a plot twist that you just did not see coming. Sometimes, you look back on a show and realize that the big twist was hinted at, but other times there's no way you could have possibly predicted what the show had in store.
While anime plot twists can be both infuriating and satisfying, they always make a show more interesting. Twists commonly involve a single character or group, but occasionally, a good series will drop in a massive twist that changes everything about the plot. When viewers find out that Assassination Classroom's Koro-Sensei never planned to destroy the Earth, the whole premise of the show suddenly changes. Even if you hated a particular plot twist, it's hard to stop thinking about them once they've dropped.
While today's Naruto fans are comfortably aware of the whole "Itachi is actually a good guy" thing, when the news first dropped, it was utterly shocking.
At the beginning of the series, Sasuke swears to take revenge on his older brother, who he believes is responsible for slaughtering their entire clan. While Itachi actually did do this, he didn't have a choice in the matter. When Itachi was 13, Danzo — a power-hungry Konoha official who secretly wanted to harvest the Uchiha clan's powerful eyes for his own use — approached him with a choice. Itachi could kill his own clan, sparing the life of himself and his brother, or the city would kill them all.
While some argue that Itachi had a third choice (warning his family and getting them out of there), that would have undoubtedly sparked another war. Itachi was already traumatized from years of warfare, and felt that killing his family was the only way to protect his community. Tragically, Sasuke doesn't learn the truth until after he exacts his vengeance and kills his brother.
The premise of Assassination Classroom is that class 3-E — a group of middle school students who are constantly looked down on for being "low level"— are tasked with assassinating their teacher, a tentacle monster who claims to have destroyed the moon, and who allegedly has plans to destroy the Earth, unless someone stops him.
In a surprising twist, it turns out that Koro-sensei didn't blow up the moon, and has no intention whatsoever of destroying the Earth. In reality, he intends to die no matter what, because his body is infused with powerful antimatter that will explode and take out most of the planet if nothing is done.
Koro-sensei asked 3-E to try and take him out for two reasons. First off, he wanted to spend his last months on Earth attempting to inspire and care for his friend dead Aguri's students. Secondly, he's genuinely hoping that they'll be able to figure out how to kill him, because he has no desire to destroy the Earth.
Most viewers of Psycho-Pass could tell that there was something weird going on with the Sybil system, but that weird thing is still one Hell of a surprise, even for the viewers who read Sybil by Flora Rheta Schreiber and therefore might have gotten the reference. The Sybil system — which decides whether or not Japanese citizens are "latent criminals" and punishes them before they can commit any crimes — is not the perfect computer algorithm that everyone believes it is.
In truth, the Sybil system is made up of a linked system of human brains, most of which were harvested from true criminals who were smart enough to outthink the system. When Akane Tsunemori discovers this, it makes her realize that she's living in a dangerously flawed society that she is now obligated to fix.
While character death doesn't automatically constitute a plot twist, L's death in Death Note is definitely unexpected. Although Light Yagami had been attempting to kill L since the beginning of the series, few people expected him to succeed, and even fewer expected that he would do so 12 episodes prior to the end of the story. Even more surprising is that, as he lays dying, L expresses genuine emotional depth; that's a twist in its own right.